Chlamydiae are important human pathogens causing a variety of ocular, respiratory and genitourinary infections, and potentially atherosclerotic cardiovascular disease. Particularly Chlamydia trachomatis is a pathogen of tremendous public health importance causing the most common bacterial sexually transmitted diseases, preventable blindness and increased risk for HIV transmission. Despite the clinical and public health importance of this organism, the mechanisms that the organism uses to interact with host cells are not understood. C. trachomatis is an obligate intracellular bacterium, developing and multiplying exclusively within the host cell in a membrane bound vacuole termed an inclusion. Research on chlamydial gene function has been hampered by the lack of laboratory methods for manipulating its genome. We propose an alternative strategy for studying chlamydial inclusion membrane proteins utilizing naturally occurring mutants and their genotypically matched wild type counterpart strains. Because the chlamydial inclusion membrane serves as the interface between the organism and the host cell, inclusion membrane proteins are therefore believed to be essential to chlamydial survival and organism-host interactions. The overall goal of this study is to utilize mutant-wild type strain pairs to assess the role of C. trachomatis IncA in interacting with the host cells. Specifically, in Aim 1 we will identify genetically closely related strains to match the naturally occurring incA mutants as an alternative to laboratory derived mutant-wild type pairs. In Aim 2, we will utilize these mutant-wild type pairs to examine the impact of Inca on host cell gene expression, especially with respect to signal transduction constituents. Results from these studies will provide new understanding of chlamydial inclusion membrane proteins and their interaction with host cells. A better understanding of chlamydial inclusion membrane proteins will provide new insights into chlamydial biology and pathogenesis.